Editing IPCC:AR6/WGIII/Chapter-14 (section)

==== 14.5.1.1 Role of Other Environmental Agreements ====

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International cooperation on climate change mitigation takes place at multiple governance levels, including under a range of multilateral environmental agreements (MEAs) beyond those of the international climate regime.

The 1987 Montreal Protocol on Substances that Deplete the Ozone Layer (the Montreal Protocol) is the leading example of a non-climate MEA with significant implications for mitigating climate change ( [[#Barrett--2008|Barrett 2008]] ). The Montreal Protocol regulates a number of substances that are both ozone-depleting substances (ODS) and GHGs with a significant global warming potential (GWP), including chlorofluorocarbons, halons and hydrochlorofluorocarbons (HCFCs). As a result, implementation of phase-out requirements for these substances under the Montreal Protocol has made a significant contribution to mitigating climate change ( [[#Molina--2009|Molina et al. 2009]] ) ( [[IPCC:Wg3:Chapter:Chapter-9#9.9.7.1|Section 9.9.7.1]] ). [[#Velders--2007|Velders et al. (2007)]] found that over the period from 1990 to 2010, the reduction in GWP100-weighted ODS emissions expected with compliance to the provisions of the Montreal Protocol was 8 GtCO 2 -eq yr –1 , an amount substantially greater than the first commitment period Kyoto reduction target. [[#Young--2021|Young et al. (2021)]] suggest that the Montreal Protocol may also be helping to mitigate climate change through avoided decreases in the land carbon sink.

The 2016 Kigali Amendment to the Montreal Protocol applies to the production and consumption of hydrofluorocarbons (HFCs). HFCs, which are widely used as refrigerants ( [[#Abas--2018|Abas et al. 2018]] ), have a high GWP100 of 14,600 for HFC-23, and are not ODS ( [[IPCC:Wg3:Chapter:Chapter-9#9.9.7.1|Section 9.9.7.1]] ). The Kigali Amendment addresses the risk that the phase-out of HCFCs under the Montreal Protocol and their replacement with HFCs could exacerbate global warming ( [[#Akanle--2010|Akanle 2010]] ; [[#Hurwitz--2016|Hurwitz et al. 2016]] ), especially with the predicted growth in HFC usage for applications like air conditioners ( [[#Velders--2015|Velders et al. 2015]] ). In this way it creates a cooperative rather than a conflictual relationship between addressing ozone depletion and the climate protection goals of the UNFCCC regime ( [[#Hoch--2019|Hoch et al. 2019]] ). The Kigali Amendment requires developed country Parties to phase down HFCs by 85% from 2011 to 2013 levels by 2036. Developing country Parties are permitted longer phase-down periods (out to 2045 and 2047), but must freeze production and consumption between 2024 and 2028 ( [[#Ripley--2016|Ripley and Verkuijl 2016]] ; [[#UN--2016|UN 2016]] ). A ban on trade in HFCs with non-Parties will come into effect from 1 January 2033. For HFC-23, which is a by-product of HCFC production rather than an ODS, Parties are required to report production and consumption data, and to destroy all emissions of HFC-23 occurring as part of HCFCs or HFCs to the extent practicable from 2020 onwards using approved technologies ( [[#Ripley--2016|Ripley and Verkuijl 2016]] ).

Full compliance with the Kigali Amendment is predicted to reduce HFC emissions by 61% of the global baseline by 2050 ( [[#Höglund-Isaksson--2017|Höglund-Isaksson et al. 2017]] ), with avoided global warming in 2100 due to HFCs from a baseline of 0.3°C–0.5°C to less than 0.1°C ( [[#WMO--2018|WMO 2018]] ). Examining the interplay of the Kigali Amendment with the Paris Agreement, [[#Hoch--2019|Hoch et al. (2019)]] show how the Article 6 mechanisms under the Paris Agreement could generate financial incentives for HFC mitigation and related energy efficiency improvements. Early action under Article 6 of the Paris Agreement could drive down baseline levels of HFCs for developing countries (calculated in light of future production and consumption in the early- and mid-2020s) thus generating long-term mitigation benefits under the Kigali Amendment ( [[#Hoch--2019|Hoch et al. 2019]] ). However, achievement of the objectives of the Kigali Amendment is dependent on its ratification by key developed countries, such as the United States, and the provision of funds by developed countries through the Protocol’s Multilateral Fund to meet developing countries’ agreed incremental costs of implementation ( [[#Roberts--2017|Roberts 2017]] ). The Kigali Amendment came into force on 1 January 2019 and has been ratified by 118 of the 198 Parties to the Montreal Protocol.

MEAs dealing with transboundary air pollution, such as the Convention on Long-Range Transboundary Air Pollution (CLRTAP) and its implementing protocols, which regulate non-GHGs like particulates, nitrogen oxides and ground-level ozone, can also have potential benefits for climate change mitigation ( [[#Erickson--2017|Erickson 2017]] ). Studies have indicated that rigorous air quality controls targeting short-lived climate forcers, like methane, ozone and black carbon, could slow global mean temperature rise by about 0.5°C by mid-century ( [[#Schmale--2014|Schmale et al. 2014]] ). Steps in this direction were taken with 2012 amendments to the CLRTAP Gothenburg Protocol (initially adopted in 1999) to include black carbon, which is an important driver of climate change in the Arctic region ( [[#Yamineva--2018|Yamineva and Kulovesi 2018]] ). The amended Protocol, which has 28 Parties including the US and EU, entered into force in October 2019. However, its limits on black carbon have been criticised as insufficiently ambitious in light of scientific assessments ( [[#Khan--2018|Khan and Kulovesi 2018]] ). There is still a non-negligible uncertainty in the assessment of radiative forcing of each short-lived climate forcer (SLCF), and the results of AR6 WGI have been updated since AR5. For example, the assessment of Emission-based Radiative Forcing from Black Carbon emissions was revised downward in AR6 (AR6 WGI [[IPCC:Wg3:Chapter:Chapter-6#6.4.2|Section 6.4.2]] ). When discussing co-benefits with MEAs related to transboundary air pollution, attention should be paid to the uncertainty in radiative forcing of SLCFs and the update of relevant scientific knowledge.

Another MEA that may play a role in aiding climate change mitigation is the 2013 Minamata Convention on Mercury, which came into force on 16 August 2017. Coal burning for electricity generation represents the second largest source (behind artisanal and small-scale gold mining) of anthropogenic mercury emissions to air ( [[#UNEP--2013|UNEP 2013]] ). Efforts to control and reduce atmospheric emissions of mercury from coal-fired power generation under the Minamata Convention may reduce GHG emissions from this source ( [[#Eriksen--2014|Eriksen and Perrez 2014]] ; [[#Selin--2014|Selin 2014]] ). For instance, [[#Giang--2015|Giang et al. (2015)]] have modelled the implications of the Minamata Convention for mercury emissions from coal-fired power generation in India and China, concluding that reducing mercury emissions from present-day levels in these countries is likely to require ‘avoiding coal consumption and transitioning toward less carbon-intensive energy sources’ ( [[#Giang--2015|Giang et al. 2015]] ). Parties to the Minamata Convention include five of the six top global CO 2 emitters – China, the United States, the EU, India and Japan (Russia has not ratified the Convention). The Minamata Convention also establishes an Implementation and Compliance Committee to review compliance with its provisions on a ‘facilitative’ basis ( [[#Eriksen--2014|Eriksen and Perrez 2014]] ).

MEAs that require state Parties to conserve habitat (such as the Convention on Biological Diversity) or to protect certain ecosystems like wetlands (such as the Ramsar Convention on Wetlands of International Importance Especially as Waterfowl Habitat) may also have co-benefits for climate change mitigation through the adoption of well-planned conservation policies ( [[#Phelps--2012|Phelps et al. 2012]] ; [[#Gilroy--2014|Gilroy et al. 2014]] ). At a theoretical level, REDD+ activities have been identified as a particular opportunity for achieving climate mitigation objectives while also conserving tropical forest biodiversity and ecosystem services. Elements of REDD+ that promise greatest effectiveness for climate change mitigation (e.g., greater finance combined with reference levels which reduce leakage by promoting broad participation across countries with both high and low historical deforestation rates) also offer the greatest benefits for biodiversity conservation ( [[#Busch--2011|Busch et al. 2011]] ). However, actual biodiversity and ecosystem service co-benefits are dependent on the design and implementation of REDD+ programmes ( [[#Ehara--2014|Ehara et al. 2014]] ; [[#Panfil--2016|Panfil and Harvey 2016]] ), with limited empirical evidence to date of emissions reductions from these programmes ( [[#Newton--2016|Newton et al. 2016]] ; [[#Johnson--2019|Johnson et al. 2019]] ), and concerns about whether they meet equity and justice considerations ( [[#Schroeder--2014|Schroeder and McDermott 2014]] ) ( [[IPCC:Wg3:Chapter:Chapter-7#7.6.1|Section 7.6.1]] ).

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